Electronic displays often use a light gating device such as a liquid crystal display (LCD) in front of a backlight. Backlights may be as simple as a lighting element such as a fluorescent tube behind the light gating device or may be more complicated arrangements. A common arrangement is to use a lighting element such as a cold cathode fluorescent tube (CCFT) or one or more light emitting diodes (LED's) placed adjacent an edge light guide lying behind the light gating device. The light guide conducts and distributes the light over the entire light gating device.
Various types of optical films maybe positioned between the light source or light guide and the light gating device. Examples include prismatic films with the prisms directed away from the light source, prismatic films with the prisms directed toward the light source, diffusing films, and reflective polarizers. The prismatic films with the prisms on the side of the film directed toward the light gating device improve the on axis brightness of the display by causing light from the backlight to be restricted to a narrower range of angles than it would be absent the film. The prism films with the prisms facing toward the backlight are used with a special class of light guides that emit light at a large angle, typically over 70°, to the normal. These prism films are used to turn the light towards the normal. The diffusers and reflective polarizers do just what their names imply.
One known problem when using prismatic films with a light gate having a rectangular array of pixels is moiré interference that arises because of the interaction of the regular pattern of linear prisms with the rows and columns of pixels in the light gating device. Various attempts have been made to avoid this problem by arranging the prismatic films such that the prisms run at an angle other than zero or 90° with the rows and columns of pixels or by choosing a prism size such that the moiré interference has a spatial frequency sufficiently different than that of the display.